Digital display systems typically produce or modulate light as a linear function of input image data for each pixel. For an 8-bit monochromatic image data word, the input image data word ranges from 0 to 255. A value of 0 results in no light being transmitted to or produced by a pixel, 255 is the maximum intensity level for a pixel, and 128 is mid-scale light.
Pulse width modulation (PWM) schemes typically modulate a constant intensity light source in periods whose length increases by a power of two. For example, when 5 mS is available for each color of a three-color system the element on times for one 8-bit system are 20 μS, 40 μS, 80 μS, 160 μS, 320 μS, 640 μS, 1280 μS, and 2560 μS. If a given bit for a particular pixel is a logic 0, no light is transmitted to or generated by the pixel. If the bit is a logic 1, then the maximum amount of light is transmitted to or generated by the pixel during the bit period. The viewer's eye integrates the light received by a particular pixel during an entire frame period to produce the perception of an intermediate intensity level.
By their nature, PWM systems produce discrete intensity levels. One problem encountered by PWM display systems is the difficulty in creating very small intensity resolution steps. As the contrast ratio of the display system increases, it becomes much more important to create very small steps between intensity levels. While a one least significant bit (LSB) intensity step is not generally objectionable when the image being displayed is very bright, it can be very objectionable in a dim region of an image.
Unfortunately, the LSB intensity step size cannot be made arbitrarily small. Image data for each bit period must be loaded into each pixel of the display device. Very small LSB periods are limited by the amount of data that can be loaded during the frame period or portion thereof. Additionally, the display device itself has some finite response time. For example, digital micromirror devices require not only a certain amount of time to load the memory array underlying the mirror array, but also a finite amount of time to reset the mirrors and allow them to transition from one position to the next.
Another problem encountered by PWM display systems is the creation of visual artifacts that arise due to the generation of an image as a series of discrete bursts of light. While stationary viewers perceive stationary objects as having the correct intensity, motion of the viewer's eye or motion in the image can create an artifact know as PWM temporal contouring. PWM temporal artifacts are described in U.S. Pat. No. 5,619,228. PWM temporal artifacts are created when the distribution of radiant energy is not constant over an entire frame period and may be noticeable when there is motion in a scene or when the eye moves across a scene.
When the eye moves across a scene, a given point on the retina of the eye accumulates light from more than one image pixel during the eye's integration period. If the various pixels are all displaying the same intensity in the same way—the discrete bursts of light are occurring simultaneously for all pixels—the perceived pixel intensity will be correct. If the various pixels are not displaying the same intensity in the same way the eye may falsely detect bright flashes. This happens when the discrete bright periods of a first pixel are created during a first portion of the frame period and the eye then scans to a second pixel that uses the next portion of the frame period to display the light. Since the same point on the retina receives the light from the first pixel and the second pixel in rapid succession—less than the decay period of the eye—that point of the retina perceives a single pixel as bright as the sum of the first and second pixels. This PWM temporal contouring artifact appears as a noticeable pulsation in the image pixels. This pulsation is time-varying and creates apparent contours in an image that do not exist in the input image data.
PWM temporal contouring is most clearly seen when viewing a grayscale ramp that increases horizontally across an image. As the image data on each line increase from 0 on the left of the row to 255 on the right, there are several places along each row where the major bits change from a logic 0 to a logic 1. The most dramatic change is in the center of each row where one pixel has a binary value of 127, which results in the first seven bits being a logic 1, and the adjacent pixel to the right having a binary value of 128, which results in the first seven bits being a logic 0 and the most significant bit being a logic 1.
If the image data is displayed over time in order of decreasing bit magnitude, that is b7, b6, b5, b4, b3, b2, b1, and b0, a viewer scanning from left to right may see an abnormally bright region at the 127 to 128 transition. This abnormal brightness is due to the viewer's eye integrating the last half of a given frame of pixel data 127—during which all bits 6:0 are all on—with the first half of the next frame—during which bit 7 is on for the entire half-frame. The net effect of the integration of the last half of the 127-valued pixel and the first half of the 128-valued pixel is a pixel having an intensity value of 255. The same artifact occurs when the pixel data is moving and the viewer's eye is stationary, and at the lower bit transitions.
When viewed at a normal viewing distance, the PWM contouring artifact created by two adjacent pixels is very difficult, if not impossible, for the typical viewer to detect. In real images, however, the bit transitions often occur in areas having a large number of adjacent pixels with virtually identical image data values. If these large areas of similar pixels have clusters whose intensity values cross a major bit transition, the PWM contouring is much easier to detect.
One method of reducing the PWM temporal contouring artifact uses bit splitting. Bit splitting divides the long periods during which the more significant bits are displayed into two or more shorter bits and distributes them throughout the frame period. For example, an 8-bit system may divide the MSB, having a duration of 128 LSB periods, into four equal periods each requiring 32 LSB periods and distributed throughout the frame period.
Bit splitting techniques reduce most of the objectionable PWM temporal artifacts. Unfortunately, bit splitting increases the necessary bandwidth of the modulator input since some of the data must be loaded into the system multiple times during a single frame period.
Given the quantization and temporal artifacts created by PWM displays, a method and system of producing very small intensity changes and eliminating noticeable temporal artifacts is needed. The method and system ideally will provide very small intensity changes without requiring the very short bit durations that are difficult to reproduce using micromechanical spatial light modulators.